Electric motor with combined permanent and electromagnets

ABSTRACT

A rotary electric motor has a stator core ( 1 ) with salient poles ( 4, 3 ), some of them ferromagnetic and some of them permanent-magnetic, which are spaced-apart and which are all simultaneously magnetizable by means of a magnetizing winding ( 5 A,  5 B). The rotor core has either reluctance poles ( 7 ) with intervening pole gaps and constant pole pitch, or permanent-magnetic poles ( 10 ) with constant pole pitch and without intervening pole gaps, alternating poles being of opposite polarities. Each reluctance pole ( 7 ) or permanent-magnetic pole ( 10 ) has a width which over a portion of the pole corresponds to respectively one-half of or the full pole pitch and is smaller over the remaining portion so that each pole ( 7  or  10 ) has a part ( 7 B or  10 A) which projects in a direction common to all poles. The ferromagnetic and permanent-magnetic poles ( 3, 4 ) of the stator ( 1 ) have a width corresponding to the width of the narrower portion of the poles ( 7  or  10 ) of the rotor ( 2 ). The number of permanent-magnetic poles ( 3 ) and, optionally, also the number of ferromagnetic poles ( 4 ) of the stator ( 1 ) is smaller than the number of reluctance poles ( 7 ) or permanent-magnetic poles ( 10 ) of the rotor ( 2 ), the positioning of the poles being such that when the motor is running, all ferromagnetic stator poles ( 4 ) will be simultaneously positioned opposite respective rotor poles ( 7  or  10 ) and all permanent-magnetic stator poles ( 30  likewise are simultaneously positioned opposite respective rotor poles ( 7  or  10 ).

BACKGROUND OF THE INVENTION

Technical field and Prior Art

The present invention relates to a rotary electric motor which has asingle, predetermined direction of rotation and which can be controlledin respect of its supply with current and its speed by a simpleelectronic circuit. The motor is a further development of the type ofmotor described and shown in Swedish Patent Application 8802972-3 (=WO90/02437).

OBJECT OF THE INVENTION

The motor described and illustrated in the aforesaid Swedish patentapplication has been found to function very well and affords all of theadvantages recited in the application. Continued development has shown,however, that further improvements in the form of lower manufacturingcosts and/or higher motor torque can be achieved without change of themotor size, at least for some fields of use, by constructing the motorin a manner which deviates slightly from the manner described in theaforesaid Swedish patent application.

According to the present invention, these further improvements areobtained with a motor constructed according to the invention, withoutlosing any of the advantages afforded by the basic motor design, asdescribed in the aforesaid patent application.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to anumber of exemplifying embodiments thereof and also with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic, principle end view of a first exemplifyingembodiment of a motor according to the invention;

FIG. 2 illustrates in a similar fashion a second exemplifying embodimentof a motor according to the invention;

FIG. 3 is a similar illustration of a third exemplifying embodiment of amotor according to the invention;

FIG. 4 is a schematic principle partial end view of a fourthexemplifying embodiment of a motor according to the invention;

FIG. 5 is a schematic principle end view of a fifth exemplifyingembodiment of a motor according to the invention; and

FIG. 6 is a schematic developed view of the construction and thearrangement of the rotor poles in the motor illustrated in FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

As explained in Swedish Patent Application 8802972-3, the type of motordescribed therein operates in accordance with the reluctance principle.One of the two mutually rotatable parts of the motor, normally therotor, is provided with a ring of ferromagnetic reluctance polesarranged with a uniform pole pitch and uniform pole gaps, and the otherpart of the motor, normally the stator, is provided with a ring ofalternating ferromagnetic poles and permanent-magnetic poles, which aremagnetized with the aid of a magnetizing winding.

The number of permanent-magnetic poles and the number of ferromagneticpoles equal the number of reluctance poles on the rotor, such that thepole pitch of the stator is half the pitch of the rotor poles.

Furthermore, each rotor reluctance pole has a width in the rotational orcircumferential direction which over a portion of the reluctance poleequals half the pole pitch of the reluctance poles and which over theremaining portion of the pole is smaller and preferably corresponds to athird of the pole pitch of the reluctance poles, such that eachreluctance pole has a part which projects in a predetermined rotationaldirection common to all reluctance poles.

The ferromagnetic and permanent-magnetic stator poles have a width asmeasured in the direction of rotation which essentially equals thereduced width of the rotor reluctance and are magnetically divided intogroups of mutually adjacent ferromagnetic and permanent-magnetic poles,with the same number of each pole type in each group. Allpermanent-magnetic poles within each such magnetic pole group have thesame permanent-magnetic polarity and all of the permanent-magnetic andthe ferromagnetic poles are magnetized in one and the same directionthrough the influence of the magnetizing winding.

With this design, the motor has only one, predetermined start direction,but current can be delivered to the motor and its speed controlled bymeans of a very simple and inexpensive electronic supply circuit. Motorsof this design can therefore be used economically for purposes for whichit has not earlier been possible to use electronically controlledmotors, for reasons of economy.

As before mentioned, the present invention is a further development ofthe tape of motor described and illustrated in Swedish PatentApplication 8802972-3 (=WO 90/02437), and is primarily based on therealization that in this kind of motor the primary purpose of thepermanent-magnetic poles is to enable the motor to start independently.

This requires the number of permanent-magnetic poles present and/or thestrength of these poles to be sufficient to overcome the static frictionof the motor and the object driven thereby when the permanent-magnetsalone, i.e. when no current flows through the magnetizing winding, drawthe rotor from a position it has adopted under the influence of thecurrent-conducting magnetizing winding, this position being referred toas the “indrawn position” in the Swedish Patent Application 8802972-3(=WO 90/02437), to a position from which a working winding through whichmagnetizing current again flows is able to rotate the rotor in thepredetermined direction. This latter position is referred to as the“start position” in the aforesaid Swedish patent application.Admittedly, the permanent-magnetic poles assist in generating a certainamount of motor torque, since the work developed by a permanent-magneticpole when attracting a reluctance pole is greater than the workdeveloped when releasing the same reluctance pole. However, it is theaforesaid independent starting of the motor in the said predeterminedstart direction which is the primary and absolutely necessary purpose ofthe permanent-magnetic poles of the motor.

The present invention is based on the realization that because of theaforedescribed circumstance, the number of permanent-magnetic polesprovided, or the strengths of said Doles, can be reduced when a motor ofthis type is used to drive loads which have low starting torques. Inview of the comparatively expensive material from which thepermanent-magnetic poles are made, a reduction in the number ofpermanent-magnetic poles of the motor will result in a significant costsaving.

A reduction in the number of permanent-magnetic poles, however, is stillmore advantageous from the point of view that more space can be providedfor the magnetizing winding, while keeping the dimensions of the motorunchanged, so as to enable the ampere turns of the magnetizing windingand, consequently, the torque generated by the motor, to be increasedsubstantially, without increasing the outer dimensions of the motor,despite the reduction in the number of permanent-magnetic poles.

FIG. 1 is a schematic end view of a first exemplifying embodiment of amotor based on the aforedescribed inventive principle.

The motor illustrated in FIG. 1 includes a stator 1 having aferromagnetic core, and a rotor 2 having a ferromagnetic core and anintermediate, cylindrical air gap 6. As is the case in the motorsdescribed in Swedish Patent Application 8802972-3 (=WO 90/2437), therotor 2 is provided with a number of salient ferromagnetic reluctancepoles 7, in the illustrated case four such poles, arranged in uniformspaced relationship. Each such reluctance pole 7 has a width in thedirection of rotation which over a portion of the pole correspondsessentially to one-half pole pitch and which over the remaining portionof the pole is smaller and preferably corresponds to one-third of thepole pitch. Consequently, each reluctance pole has nose 7B whichprojects in a predetermined direction of rotation, in the mannerdescribed in the aforesaid Swedish patent application.

The stator 1 is provided with four uniformly distributed salientferromagnetic reluctance poles 4, whose widths in the direction ofrotation correspond essentially to the narrower width of the rotorreluctance poles 7 and all of which will thus be located simultaneouslyopposite a rotor reluctance pole 7 when the motor runs. The stator 1 isalso provided with two diametrically opposed permanent-magnetic poles 3of mutually opposite polarities in relation to the air gap 6.

When seen magnetically, the reluctance poles 4 and the permanent-magnetpoles 3 on the stator are divided into two diametrically opposed groups,each comprising two reluctance poles 4 and an intermediatepermanent-magnetic pole 3 and each provided with an associatedmagnetizing winding 5A and 5B, respectively, for magnetizing all polesin a particular group simultaneously and in the same direction, namelypreferably in the direction which is opposite to the permanent-magneticpolarity of the permanent-magnetic pole 3.

Since the two magnetizing windings 5A and 5B are intended to car currentsimultaneously and in mutually the same direction, it will be understoodthat these windings may also be constructed in the form of a singlewinding.

It will be seen that in comparison with the motor described in SwedishPatent Application 8502972-3 (=WO 90/02437), the motor according to theinvention as illustrated in FIG. 1 lacks two permanent-magnetic statorpoles and that the space which would otherwise be occupied by thesepermanent-magnetic poles instead accommodates the magnetizing windings5A, 5B. This has enabled the total conductor cross-section of themagnetizing windings and, consequently, the magnetomotive force orampere turns of the windings, to be increased considerably. As a result,a corresponding increase in the torque generated by the motor has beenachieved which more than makes up for the reduction in motor torquecaused by the reduction in the number of permanent-magnetic statorpoles.

There is thus provided a motor whose overall dimensions remain unchangedbut which nevertheless generates a higher torque and can be manufacturedat lower cost because of the reduced number of permanent-magnetic polesprovided.

The motor according to the illustrated in FIG. 1 has, in general, all ofthe advantages afforded by a motor of the kind described and illustratedin Swedish Patent Application 8802972-3 (=WO 90/02437). This motor canbe driven and its speed controlled with the aid of a very simpleelectronic supply circuit, for instance a circuit of the kind describedin the Swedish patent application.

If the number of reluctance poles 4 in each of the two diametricallyopposed pole groups on the stator is increased in a motor of the kinddescribed above and illustrated in FIG. 1, the least possible number ofpermanent-magnetic poles 3 will still be only one in each pole group, itbeing possible to place this permanent-magnetic pole in any selected gapbetween the reluctance poles 4 in the group concerned.

The motor will have the highest torque density, however, when apermanent-magnetic pole 3 is placed in each interspace between thereluctance poles 4 in the pole group concerned, since the omission ofpermanent poles in these interspaces will not result in greater spacefor the accommodation of the magnetizing winding. Thus, in this case,the number of permanent-magnetic poles 3 per pole group on the stator 1will be equal to the number of reluctance poles 4 in each pole groupminus 1.

If the number of stator pole groups in a motor of the kind describedabove and as illustrated in FIG. 1, is increased from two to an integralmultiple of two, the smallest possible number of permanent-magneticpoles on the stator will still be two, with mutually opposite polaritiesrelative to the air gap, placed in two different pole groups. Thehighest torque density, however, will be obtained with apermanent-magnetic pole placed in each interspace between the reluctancepoles 4 in each pole group, but with no permanent-magnetic poles placedbetween the different pole groups.

In the case of a motor of the kind described above and as illustrated inFIG. 1, it is only the magnetic flux from the permanent-magnetic poles 3which passes through those parts of the back of the stator 1 to whichthe permanent-magnetic poles are attached. This does not require thecross-sectional area, i.e. the radial height, of these parts of thestator back to be as large as that if those parts of the stator backthrough which the magnetic flux generated by the working windings 5A, 5Bthrough the reluctance poles 4 flows.

Furthermore, the height of the permanent-magnetic poles 3, i.e. theirradial dimension, may advantageously be smaller than the radialdimension of the reluctance poles 4. Consequently, it is quite possibleto reduce the overall dimensions of the stator 4 over the positions ofthe two permanent-magnetic poles 3 while retaining the overall statordimensions in the direction perpendicular thereto. i.e. in the directionin which the coil sides of the working windings 5A, B are located.

Since, in the mass production of motors of the kind concerned here, thesheet metal stampings which form the stator core are produced from metalstrip, there is no reason whatsoever to reduce the cross-sectional areaavailable for the stator winding by curving the outer contours of thestator core. The outer contours of a low-price motor based on theprinciple described above and illustrated in FIG. 1 should therefore berectangular, as illustrated schematically in FIG. 2. Those parts in FIG.2 which correspond to parts shown in FIG. 1 are identified with the samereference signs.

It will be seen that the FIG. 2 embodiment affords still greater spacefor the working windings 5A, 5B than the FIG. 1 embodiment, andconsequently enables the working winding to have a still higher ampereturns, thereby increasing the torque generated by the motor.

A comparison of a motor having a stator constructed in accordance withFIG. 1 and a motor having a stator constructed in accordance with FIG.2, but with identical rotors and stators of equal overall dimensions inthe direction extending perpendicular to the line along which thepermanent-magnetic poles 3 are placed, shows that a motor having thestator stamping shape shown in FIG. 2 requires about 20% less metalsheet for stator manufacture and is able, at the same time, to develop atorque which is about 20% greater than the torque developed by the motorillustrated in FIG. 1.

A further development of the present invention is based on therealization that the number of ferromagnetic reluctance poles 4 on thestator coacting with the magnetizing winding or working winding can bereduced so that the number of these reluctance poles 4 will be smallerthan the number of rotor reluctance poles 7. One condition in thisrespect, however, is that the stator reluctance poles 4 coacting withthe working winding are still arranged so that all of the poles 4 willbe located simultaneously essentially opposite a respective rotorreluctance pole 7 as the motor runs. FIG. 3 illustrates schematicallyand by way of example a motor which is constructed in accordance withthis principle and which can be considered a modification of the motorillustrated in FIG. 2.

In the case of the motor illustrated in FIG. 3, the rotor 2 is providedwith six reluctance poles 7 which are formed with projecting parts ornoses 7B in the afore-described manner, whereas the stator 1 is providedwith two diametrically opposed groups of magnetic poles, each includingtwo ferromagnetic reluctance poles 4 and an intermediatepermanent-magnetic pole 3, similar to the motor illustrated in FIG. 2.

Thus, in comparison with a motor according to the aforesaid SwedishPatent Application 8802972-3 (=WO 90/02437), the motor illustrated inFIG. 3 lacks two ferromagnetic reluctance poles and fourpermanent-magnetic poles on the stator 1. It will be realizedimmediately from the FIG. 3 illustration that this results in stillgreater space 8 for accommodating the magnetizing or working windings,which are not shown in FIG. 3.

When a motor constructed in accordance with FIG. 3 is compared with amotor constructed in accordance with FIG. 2 and it is assumed that thesurfaces of the reluctance poles 4, 7 which face the air gap are of thesame size in both cases and that the magnetizing or working windingsdevelop magnetomotive force of equal magnitudes in both directions, amotor constructed in accordance with FIG. 3 will generate a torque whichis 1.5 times greater than the torque generated by a motor which isconstructed in accordance with FIG. 2.

At the same time, the motor constructed in accordance with FIG. 3requires a 50% higher current supply frequency at the same speed. Themotor illustrated in FIG. 3 can therefore be equated with a motoraccording to FIG. 2 that is provided with a mechanical speedtransmission having a transmission ratio of 3:2.

A motor constructed in accordance with FIG. 3 may be advantageous in thecase of relatively low-speed applications in which the increase incurrent supply frequency required by the FIG. 3 embodiment in comparisonwith the FIG. 2 embodiment can be accepted, both with respect to theiron losses in the motor and in the switch frequency in the electronicsupply circuit of the motor.

In the case of motors provided with a rectangular stator core, forexample in accordance with FIG. 2 or FIG. 3, the magnetizing windingsmay be given the form of coil windings instead of bundle or skeinwindings. These coil windings may be arranged around those sides (legs)of the stator core 1 which connect the two diametrically opposed polegroups 3, 4. In the case of very small motors, these coil windings maybe mounted on circuit boards, optionally together with the electronicsupply circuit, so as to obtain a flat electronic motor.

The stator core 1 of small motors may also be configured with solely oneside (leg) connecting the two diametrically opposed pole groups 3, 4, sothat the stator core 1 will have a generally C-shaped cross-section. Ofcourse, the one remaining side (the leg) of the stator 1 and also theportions of the core which support the poles 3, 4 must be given a largercross-sectional area for the magnetic flux than in the case of theembodiments according to FIGS. 2 and 3. In a motor of this kind, it issufficient to provide one single magnetizing winding in the form of acoil winding arranged around the single side (leg) of the stator core 1.

An extreme, conceivable variant of the type of motor illustrated in FIG.3 is one in which the number of rotor reluctance poles 7 is many timesgreater than the number of stator poles 3, 4. In a motor of this kind,the stator may suitably have the form of a so-called short stator whichextends over only a small part of the rotor circumference, i.e. does notsurround the rotor.

FIG. 4 illustrates schematically and in partial end view an exemplifyingembodiment of such a motor. In this case, it is necessary for the shortstator 9 to include at least two pole groups, each including at leasttwo ferromagnetic reluctance poles 4 and at least one permanent-magneticpole 3. In this embodiment, the magnetizing or working winding 5 is acoil winding which embraces that part of the stator core 9 whichconnects the two pole groups.

Several such short stators may be arranged to coact with one and thesame rotor, for example two short stators may be mounted indiametrically opposed relationship. Extreme variants of a short- statormotor include a linear motor or a motor whose rotor is comprised of onlyone arcuate segment along whose circumference the reluctance poles thathave outwardly projecting noses are placed.

As before mentioned, the principle of the present invention lies in thereduction of the number of permanent-magnetic poles and optionally alsothe number of ferromagnetic reluctance poles on the motor-part that isprovided with the magnetizing or working winding, so that the number ofpoles of this kind will be smaller than the number of reluctance polesprovided with a projecting nose and mounted on the other motor part.

This principle can also be applied to types of motors that have anaxial-radial magnetic flux path, i.e. a motor of the kind illustrated inFIGS. 5-7 of the aforementioned Swedish Patent Application 8802972-3(=WO 90/02437). This enables, in principle, the number ofpermanent-magnetic poles to be reduced equally in each of the twoaxially spaced, cylindrical air gaps. However, a permanent-magnetic polemust be present at each air gap.

The reduction in the number of permanent-magnetic poles also results,however, in a reduction in the torque generated by the motor, as beforementioned, which in this case can be compensated for by axiallyextending the magnetizing winding, which has the form of a coil winding.

A reduction in the number of permanent-magnetic poles lowers themanufacturing costs of the motor. As before mentioned, it is possiblewith this type of motor to reduce the number of ferromagnetic reluctancepoles on the stator, so that there will be fewer stator reluctance polesthan those rotor reluctance poles that have an outwardly projectingnose.

However, since a reduction in the number of ferromagnetic statorreluctance poles in this motor construction will not increase the spaceavailable for the magnetizing winding on the stator, there is normallyno reason to reduce the ferromagnetic stator reluctance poles to anumber which is smaller than the number of rotor reluctance poles.

In the case of a motor having an axial-radial magnetic flux path, it isalso conceivable to form one of the two axially spaced, cylindrical airgaps without providing any poles at all on either the rotor or thestator, i.e. with mutually facing, smooth surfaces on the rotor core andthe stator core, the smooth air gap being dimensioned in a manner tooffer relatively small reluctance to the passage of the magnetic fluxthat is determined by the configuration of the magnetic poles on thestator and the rotor at the second air gap.

Such an embodiment with only one air gap that is provided with poles anda smooth air gap is possible both in a motor construction according tothe present invention and with a motor construction according to theaforesaid Swedish Patent Application 8802972-3 (=WO 90/02437), i.e.irrespective of whether the number of permanent-magnetic poles andferromagnetic reluctance poles on one motor part is the same as orsmaller than the number of reluctance poles on the other motor partwhich are provided with noses.

A further development of the present invention is based on therealization that the reluctance poles on one motor part which areprovided with noses can be replaced with similarly configuredpermanent-magnetic poles. This would be particularly advantageous in thecase of very small motors, because when the dimensions of a motordecrease, the magnetomotive force that can be developed by the workingwinding at permitted winding temperatures also decreases.

For example, if the motor diameter were to be halved, only about 35% ofthe earlier magnetomotive force would be available, because the air gapbetween rotor and stator cannot be reduced to a corresponding extent,for mechanical reasons. This fact restricts the use of the reluctanceprinciple in very small motors. Consequently, the use ofpermanent-magnetic poles is preferred in very small motors, particularlysince the cost of permanent-magnetic poles is low as a result of thesmall amount of permanent-magnetic material consumed.

FIGS. 5 and 6 illustrate schematically, and by way of example, a motoraccording to the invention constructed in the aforesaid manner. FIG. 5is an end view of the motor and FIG. 6 is a developed view of theconstruction and arrangement of the permanent-magnetic poles mounted onthe rotor.

The construction of the stator 1 with its ferromagnetic reluctance poles4 and permanent-magnetic poles 3 and magnetizing or working windings 5A,5B corresponds to the construction of the motors earlier described withreference to FIGS. 2 and 3. However, in the case of the embodimentsillustrated in FIGS. 5, 6, the magnetizing windings 5A, 5B are intendedto generate a magnetic flux which coincides with the polarity of thepermanent-magnetic poles 3, whereas in the case of the earlier describedembodiments with reluctance poles provided on the rotor, the conversecondition is applied to advantages.

As before mentioned, in the case of the motor construction illustratedin FIGS. 5, 6, the rotor reluctance poles provided with noses have beenreplaced with permanent-magnetic poles configured and arranged in themanner best seen from FIG. 6. This means, in principle, that eachreluctance pole that is provided with a nose, together with the pole gapbetween it and the next-following reluctance pole, has been replacedwith a pair of permanent-magnetic poles 10 of mutually oppositepolarities and configured with projecting parts or noses 10A in thedesired direction of rotation.

The narrower width of each such permanent-magnetic pole 10 correspondsgenerally with the width of the ferromagnetic reluctance poles 4 and thepermanent-magnetic poles 3 of the stator, whereas the greater width ofthe permanent-magnetic poles 10 corresponds with the pole pitch of thepoles 3, 4 on the stator 1 and, consequently, also with the pole pitchof the permanent-magnetic poles 10 on the rotor.

The motor illustrated in FIGS. 5, 6 operates in the following manner.When no current passes through the stator windings 5A, 5B, the rotor 2will position itself in a position in which the statorpermanent-magnetic poles 3 will attract rotor permanent-magnetic poles10 of opposite polarity. The attracted rotor permanent-magnetic poles 10will then take a position with maximum overlap with the statorpermanent-magnetic poles 3.

When current is passed through the stator windings 5A, 5B, the statorreluctance poles 4 will attract the rotor permanent-magnetic poles 10 ofopposite polarity, the projecting noses 10A of these poles 10 beinglocated adjacent the stator reluctance poles concerned. Provided thatthe pulling force exerted by the reluctance poles 4 is greater than theretaining force exerted on the rotor 2 by the permanent-magnetic poles 3of the stator, the rotor will move through one pole pitch in thedirection of the projecting noses 10A of the permanent-magnetic poles 10of the rotor 10.

When the attracted poles 10 on the rotor have reached approximatelymaximum overload with the stator reluctance poles 4, the current to thestator windings 5A, 5B is switched off by the magnetizing system, notshown. In this operational state of the motor, the permanent-magneticpoles 3 of the stator are overlapped by those permanent-magnetic poles10 on the rotor 2 which repel the permanent-magnetic poles 3 on thestator. Those rotor poles 10 which have opposite polarity and theprojecting noses 10A of which are located adjacent thepermanent-magnetic poles 3 of the stator 1 are then attracted by thesepoles and will move through one pole pitch in the direction of theoutwardly projecting noses 10A of the motor poles 10.

It will be noted that the permanent-magnetic poles 10 on the rotor neednot be separated physically, and that they can be provided,advantageously, by magnetizing a ring of permanent-magnetic materialaround the periphery of the ferromagnetic core of the rotor. Thistechnique is known per se.

The principle that has been applied in the construction of the motorillustrated in FIGS. 5, 6, namely the principle of replacing rotorreluctance poles that have projecting noses with permanent-magnets thathave projecting noses, may also be applied to the type of motordescribed in the Swedish Patent Application 8802972-3 (=WO 90/02437).

In the case of an electric motor stator provided with a working winding,efforts are always made to achieve the best possible balance betweenthat part of the total cross-sectional area of the stator which isoccupied by ferromagnetic material for conducting the magnetic flux andthe remainder of the cross-sectional area available for accommodatingthe working winding. This is so, because it is the product of the changein the magnetic flux, on the one hand, and the magnetomotive force ofthe working winding caused by rotor movement, on the other hand, that isproportional to the motor torque and which should therefore bemaximized.

In the case of the motor according to the present invention, or a Motorconstructed in accordance with Swedish Patent Application 8802972-3,which have a magnetic circuit constructed from parallel, sheet metalstampings, the aforesaid maximization provides a best interval for poleflux magnitude which corresponds, in general, with a value of themagnetic flux density in the overlap area between the coacting poleswhich is below the value at which magnetic saturation occurs in themagnetic steel sheets.

To ensure that the torque developed by the motor will be as uniform aspossible, it is endeavoured to change the pole flux essentially inproportion to the change in the overlap area between the mutuallycoacting stator and rotor poles. This requires that the change in fluxwill not be limited by magnetic saturation in any other part of themagnetic circuit, i.e. that the magnetic flux density in the overlaparea between the mutually coacting poles can be kept at a generallyconstant level.

For a given value of the magnetomotive force which acts over the overlaparea between stator poles and rotor poles, this can be achieved eitherby selecting a sufficiently large air gap between rotor and stator polesor by giving the stator pole surfaces and/or the rotor pole surfaces anature such as to restrict flux density by magnetic saturation in thesesurfaces. This latter method is preferred, primarily because the torquedeveloped is greater than that developed with the first mentionedmethod.

The simplest method of achieving magnetic saturation of the pole surfacelayers is to thin-out the plate pack in the vicinity of the polesurface, for instance by terminating every second or third plate a fewmillimeters from the pole surface, while continuing with the remainingplates right up to said pole surface. This method of construction can beused advantageously with the ferromagnetic reluctance poles on the rotorand/or the stator of a motor constructed in accordance with the presentinvention or a motor constructed in accordance with the earliermentioned Swedish patent application.

I claim:
 1. An electric motor comprising a first motor part and a secondmotor part (1, 2) which are rotatable relative to one another and eachof which has a ferromagnetic core and which are separated by an air gap(6) lying between the ferromagnetic cores, wherein the ferromagneticcore of said first motor part (1) has provided on the surface thereofwhich faces the air gap (6) a plurality of salient magnetic poles whichare disposed sequentially and in spaced relationship in the direction ofrotation, and of which some are ferromagnetic (4) and some arepermanent-magnetic (3) and all of which are magnetically connected to amagnetizing winding (5A, 5B) for simultaneous magnetization of both theferromagnetic (4) and permanent-magnetic (3) poles, wherein theferromagnetic core of the said other motor part (2) is provided on thesurface thereof facing the air gap (6) and opposite the magnetic poles(3, 4) on the core of the first part (1) with a row of salientferromagnetic reluctance poles (7) which extend are arranged in a rowextending in said direction of rotation and which have a constant polepitch and are spaced-apart equidistantly, wherein each such reluctancepole (7) over a portion thereof has a width in the direction of rotationwhich corresponds essentially to half said pole pitch and over theremaining portion thereof is smaller such that each reluctance pole (7)has a part comprises a first portion and a second portion (7B) whichprojects in a predetermined direction common to all reluctance poles(7), wherein the ferromagnetic and permanent-magnetic poles (3, 4) onthe core of the first motor part (1) have a width in the direction ofrotation which corresponds essentially to the width of the narrow partfirst portion of the reluctance poles (7) and are so positioned thatduring relative rotation of the two motor parts (1, 2), allferromagnetic poles (4) will be located simultaneously opposite theirrespective reluctance poles (7), on the second motor part (2) and,similarly, such that all permanent-magnetic poles (3) will besimultaneously located opposite their respective reluctance poles (7),and wherein the number of permanent-magnetic poles (3) is smaller thanthe number of reluctance poles (7) on the core of the second motor part(2), such that no permanent-magnetic poles (3) are present in some ofthose positions which would be occupied by such permanent-magnetic polesif a number of permanent-magnetic poles (3) equal to the number ofreluctance poles (7) of the core of the second motor part (2) weredistributed uniformly.
 2. A motor according to claim 1, wherein thenumber of ferromagnetic poles (4) on the core of the first motor part(1) is smaller than the number of reluctance poles (7) on the core ofthe second motor part (2), such that no ferromagnetic poles (4) arepresent on the core of the first motor part (1) at a number of thosepositions in which such ferromagnetic poles would be present withuniform distribution of a number of ferromagnetic poles (4) equal to thenumber of reluctance poles (7) present on the core of the second motorpart (2).
 3. A motor according to claim 1, wherein those spaces on thecore of the first part (1) in which no permanent-magnetic and/orferromagnetic poles (3, 4) are present are used to accommodate themagnetizing winding (5A,5B).
 4. A motor according to claim 1 having acylindrical air gap (6), wherein the first motor part (1) forms a statorwhich lies outside the air gap (6) and the second motor part forms arotor (2) which lies inwardly of the air gap (6), wherein stator core(1) is provided with an even number of magnetic pole groups; in thateach such pole group includes at least two ferromagnetic poles (4)having a pole pitch which corresponds to the pole pitch of thereluctance poles (7) on the rotor core (2), at least two such polegroups additionally including at least one ferromagnetic pole (3) placedin the gap between two ferromagnetic poles (4) of the respective polegroup; and in that the two permanent-magnetic poles (3) have mutuallyopposed polarities relative to the air gap (6) and each pole group is somagnetically connected to he magnetizing windings (5A, 5B) that all ofthe poles (3, 4) in the pole group are magnetized in mutually the samedirection but in opposite directions relative to adjacent pole groups.5. A motor according to claim 4, wherein the stator core (1) has agenerally rectangular cross-section and carries on each of two opposingsides a pole group which includes at least two ferromagnetic poles (4)and at least one permanent-magnetic pole (3) placed in the gaptherebetween.
 6. A motor according to claim 5, wherein the magnetizingwinding is comprised of coil windings arranged around both other sidesof the stator core (1).
 7. A motor according to claim 4, wherein thestator core (1) has a generally C-shaped cross-section and carries oneach of its two opposing legs a pole group which includes at least twoferromagnetic poles (4) and at least one permanent-magnetic pole (3)placed in the gap therebetween; and in that the magnetizing winding iscomprised of a coil winding arranged around the centre part of thestator core connecting the two legs of the stator core (1).
 8. A motoraccording to claim 2 having a cylindrically curved air gap, wherein thefirst motor part forms a stator which lies outside the air gap and thesecond motor part forms a rotor which lies inwardly of the air gap,wherein the stator core (9) extends along only a limited part of thelength of the air gap as seen in the direction of relative rotation andis provided with at least two groups of magnetic poles each includingtwo ferromagnetic poles (4) and a permanent-magnetic pole (3) placed inthe gap therebetween; in that the permanent-magnets (3) in the two polegroups have mutually opposite polarities in relation to the air gap; andthat the magnetizing winding (5) is comprised of a coil winding arrangedaround that part of the stator core (9) which magnetically connects thetwo pole groups.
 9. A motor according to claim 8, wherein the motorincludes a plurality of stator cores (9) of the aforesaid constructionarranged in different positions along the rotor core (2).
 10. A motoraccording to claim 1, wherein the ferromagnetic cores of the two motorparts are separated by two cylindrical, coaxial and axially spaced airgaps; in that the cores of the two motor parts are provided withferromagnetic and permanent-magnetic poles and reluctance polesrespectively in the aforesaid manner at at least one air gap; and inthat the magnetizing winding is comprised of a coil winding arranged,when seen axially, between said two air gaps.
 11. A motor according toclaim 1, wherein the cores of the two motor parts are also provided withferromagnetic and permanent-magnetic poles and reluctance polesrespectively in the aforesaid manner at the second air gap; and in thatthe permanent-magnetic poles have oppositely directed polarities at thetwo air gaps.
 12. A motor according to claim 11, wherein the cores ofthe two motor parts have smooth surfaces on which no poles are presentat the second air gap.
 13. A motor according to claim 1, wherein thereluctance poles on the core of the second motor part (2) are replacedwith permanent-magnetic poles (10) in a manner such that instead of eachreluctance pole, there are two permanent-magnetic poles (10) which arelocated adjacent one another in the direction of a rotation and whichhave mutually opposite polarities; in that each such permanent-magneticpole (1) has a width in the direction of rotation which over a portionof the pole is equal to half the pole pitch and over the remainder ofthe pole is narrower so that each permanent-magnetic pole (10) has apart (10A) a first portion and a second portion ( 10A) each of whichprojectsproject in a direction common to all poles; and in thepermanent-magnetic poles (3) on the core of the first motor part (1) areintended to be magnetized by the magnetizing winding (5A, 5B) in adirection which coincides with its permanent-magnetic polarity.
 14. Amotor according to claim 2, wherein those spaces on the core of thefirst part (1) in which no permanent-magnetic and/or ferromagnetic poles(3, 4) are present are used to accommodate the magnetizing winding(5A,5B).
 15. A motor according to claim 14 having a cylindrical air gap(6), wherein the first motor part (1) forms a stator which lies outsidethe air gap (6) and the second motor part forms a rotor (2) which liesinwardly of the air gap (6), wherein the stator core (1) is providedwith an even number of magnetic pole groups; in that each such polegroup includes at least two ferromagnetic poles (4) having a pole pitchwhich corresponds to the pole pitch of the reluctance poles (7) on therotor core (2), at least two such Dole groups additionally including atleast one ferromagnetic pole (3) placed in the gap between twoferromagnetic poles (4) of the respective pole group; and in that thetwo permanent-magnetic poles (3) have mutually opposed polaritiesrelative to the air gap (6) and each pole group is so magneticallyconnected to the magnetizing windings (5A, 5B) that all of the poles (3,4) in the pole group are magnetized in mutually the same direction butin opposite directions relative to adjacent pole groups.
 16. A motoraccording to claim 15, wherein the stator core (1) has a generallyrectangular cross-section and carries on each of two opposing sides apole group which includes at least two ferromagnetic poles (4) and atleast one permanent-magnetic pole (3) placed in the gap there between.17. A motor according to claim 16, wherein the magnetizing winding iscomprised of coil windings arranged around both other sides of thestator core (1).
 18. A motor according to claim 2, wherein theferromagnetic cores of the two motor parts are separated by twocylindrical, coaxial and axially spaced air gaps; in that the cores ofthe two motor parts are provided with ferromagnetic andpermanent-magnetic poles and reluctance poles respectively in theaforesaid manner at at least one air gap; and in that the magnetizingwinding is comprised of a coil winding arranged, when seen axially,between said two air gaps.
 19. A motor according to claim 18, whereinthe reluctance poles on the core of the second motor part (2) arereplaced with permanent-magnetic poles (10) in a manner such thatinstead of each reluctance pole, there are two permanent-magnetic poles(10) which are located adjacent one another in the direction of arotation and which have mutually opposite polarities; in that each suchpermanent-magnetic pole (10) has a width in the direction of rotationwhich over a portion of the pole is equal to half the pole pitch andover the remainder of the pole narrower so that each permanent-magneticpole (10) has part (10A) a first portion and a second portion ( 10A)each of which projectsproject in a direction common to all poles; and inthat the permanent-magnetic poles (3) on the core of the first motorpart (1) are intended to be magnetized by the magnetizing winding (5A,5B) in a direction which coincides with its permanent-magnetic polarity.20. A motor according to claim 16, wherein the reluctance poles on thecore of the second motor part (2) are replaced with permanent-magneticpoles (10) in a manner such that instead of each reluctance pole, thereare two permanent-magnetic poles (10) which are located adjacent oneanother in the direction of a rotation and which have mutually oppositepolarities; in that each such permanent-magnetic pole (10) has a widthin the direction of rotation which over a portion of the pole is equalto half the pole pitch and over the remainder of the pole is narrower sothat each permanent-magnetic pole (10) has a part (10A) a first portionand a second portion ( 10A) each of which projectsproject in a directioncommon to all poles; and in that the permanent-magnetic poles (3) on thecore of the first motor part (1) are intended to be magnetized by themagnetizing winding (5A, 5B) in a direction which coincides with itspermanent-magnetic polarity.